1. Field of the Invention
The present invention relates to arrangements for providing movement of the seat of an agricultural tractor or similar vehicle to cushion the operator from shocks and other undesired motions, and more particularly to an active seat suspension system in such vehicles in which the seat is actively caused to move in synchronism with motion of the vehicle so as to compensate for such motion.
2. History of the Prior Art
It is known to utilize an active seat suspension in an agricultural tractor or similar vehicle which undergoes substantial off-road use or otherwise encounters the type of terrain which subjects the vehicle and the operator thereof to significant jolts and bumps and other undesired motions. An active seat suspension system responds to motions of the vehicle by actively moving the vehicle seat in a direction and by an amount so as to compensate for such motions. Such systems are different from the more common passive systems such as those which attempt to damp or attenuate movements of the seat in horizontal or vertical directions.
An example of an active seat suspension system for tractor seats and the like is provided by an article entitled "Curing The Tractor Seat Shakes: How About Electrohydraulics?" by Suggs and Stikeleather on pp. 24 and 25 of the Oct. 7, 1971 issue of Implement and Tractor. This article which relates to ASEA Paper No. 68-632 by L. F. Stikeleather and C. W. Suggs published in December of 1968 and an article in the same year by Larry Franklin Stikeleather entitled "The Development Of An Active Seat Suspension System For Off-Road Vehicles" from the Department of Biological and Agricultural Engineering of North Carolina State University at Raleigh, generated in connection with a graduate thesis, describes an active suspension system in which a vibrometer and electrical bridge circuit are used to adjust the height of a tractor seat in response to tractor movement sensed by the vibrometer. The Implement and Tractor article notes the undesirability of low frequency vibration and at the same time the suitability of a vibrometer for sensing such low frequency vibrations so as to position the height of the seat using an active suspension system. Other examples of seat or platform suspension systems for both agricultural and other applications are provided by U.S. Pat. Nos. 4,198,025 to Lowe et al, 3,951,373 of Swenson et al, 3,701,499 of Schubert et al and 3,233,886 of Saffell et al.
An active suspension system of the type described in the above-noted articles by Suggs and Stikeleather has considerable attractiveness from a theoretical standpoint. Assuming the vibrometer is able to follow the low frequency motions undergone by the tractor, it is only necessary to design a servo system which causes the seat to move vertically in an out-of-phase relationship with the low frequency motions of the tractor. The ideal active suspension system would move the seat in a vertical direction relative to the tractor as necessary so as to maintain the seat in a horizontal plane as the tractor moves up and down in response to bumps or otherwise varying terrain which causes the low frequency motions. However, while such a system has much attractiveness from a theoretical standpoint, actually building such a system presents a number of practical problems which are not acknowledged, let alone solved, by the above-noted articles.
One such problem relates to a type of safety problem inherent in most active suspension systems. Thus, so long as the vibrometer or other sensing instrument is capable of accurately following vehicle motion, the seat will be positioned in such a way as to optimize operator comfort. However, situations may occur where the vibrometer becomes incapable of following the vehicle motion. For example, in the case of extreme bumps or jolts the suspended mass or other moving member within the vibrometer may be incapable of traveling the distance required at the speed required so as to accurately follow the vehicle motion. A related problem relates to the fact that the seat and the active suspension therefor cannot move beyond a predetermined range. As a result, in the case of extreme bumps or jolts the vertical seat movement may become in-phase with the vehicle movement so that the seat begins to move in the wrong direction. When this occurs the active suspension system acts to amplify rather than attenuate the effects of vehicle motion at the seat. Such results are disturbing at the very least and can become so dangerous as to cause the operator to be thrown from the seat or to lose control of the vehicle.
Further problems relates to the desirability of varying such things as the sensitivity and stiffness of the system in addition to the initial height of the seat. For certain conditions of operation it may be desirable to make the suspension system highly sensitive. At the same time high sensitivity may not be desired for other conditions of operation, and particularly when the active suspension system is first turned on. It is preferable to allow the operator at least some time to become accustomed to the suspension system before a state of high responsiveness is entered. It is also generally desirable to be able to vary the rate at which the seat is moved in response to the vibrometer, and thereby the stiffness of the suspension system. Different types of rides and bumpy conditions dictate different degrees of stiffness desired in the suspension system for optimum operator comfort. Another desirable feature would be the ability to initially adjust the height of the seat electronically from the control circuit where desired.
Vibrometers are difficult to find in commercially available form, and those which are so available tend to be unsuited for use in an active tractor seat suspension system. The consequent custom designing of a vibrometer for such applications is difficult and expensive and frequently results in an arrangement which is impractical and of unwieldy size.
Accordingly, it is an object of the invention to provide an improved active seat suspension system including the control system therefor.
It is a further object of the invention to provide an active seat suspension control system which is automatically disabled when either of opposite extremes of motion of the vibrometer is encountered.
It is a still further object of the invention to provide an active seat suspension control system providing for the adjustment of sensitivity, stiffness and initial seat height electronically.
It is a still further object of the invention to provide an improved vibrometer ideally suited for use in an active seat suspension control system with agricultural tractors and similar vehicles.